Li-ion/Li-polymer battery charger configured to be DC-powered from multiple types of wall adapters

ABSTRACT

A battery charger controller is coupled to DC output terminals of an AC-DC (or DC-DC) adapter containing an AC-DC (or DC-DC) converter. A controlled current flow path between input and output terminals of the battery charger controller circuit is controlled to provide a substantially constant current to charge the battery to a nominal battery voltage. When a constant voltage output of the said adapter transitions to a value that limits available charging current to a value less than programmed constant charging current, current flow drive for the controlled current flow path is increased for a limited time interval. Thereafter, the controlled current flow path gradually reduces charging current as the battery voltage remains at its nominal battery voltage until the charge is complete or otherwise terminated.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation application of U.S. patentapplication Ser. No. 10/314,543, filed on Dec. 9, 2002, entitled“Li-Ion/Li-Polymer Battery Charger Configured To Be DC-Powered FromMultiple Types Of Wall Adapters”, by Lai et al, now U.S. Pat. No.6,844,705, assigned to the assignee of the present application, and thedisclosure of which is incorporated herein.

FIELD OF THE INVENTION

The present invention relates, in general, to battery chargers,including those used for charging DC batteries/cells, such as, but notlimited to, Li-ion/Li-polymer batteries of the type used to powerportable electronic devices, such as laptop/notebook computers, personaldigital assistants (PDAs), and the like, and is particularly directed toa new and improved battery charger controller architecture having‘plug-in’ compatibility with various types of power adapters, whileproviding substantially reduced thermal dissipation.

BACKGROUND OF THE INVENTION

Rechargeable, single-cell batteries, such as Li-ion/Li-polymerbatteries, are becoming commonplace DC power supply cells for a varietyof portable and handheld products. As one would expect, the demand forincreased functionality and longer run time of such battery-poweredproducts has resulted in a demand for increased battery cell capacity,with an attendant increase in power required to charge them. A typicalsingle-cell battery charger controller has a relatively compact andportable arrangement, as diagrammatically illustrated in FIG. 1. Asshown therein, the charger system includes an external power adapter 10,having an external power pair that is configured to be plugged into asource of external power, such as a 110 VAC wall outlet or automobileelectrical system, and a power cable connector 12 that mates with aconnector receptacle 22 of a DC-DC converter/charging unit 20. Thecharger controller unit proper is configured to maintain a battery 30 tobe charged.

At present, the majority of DC-DC converter/charging units of suchportable battery charger arrangements are based upon a linear transferfunction design, such as that diagrammatically shown in FIG. 2. In alinear charger, the wall adapter 10 serves as a DC voltage source andtypically has a substantially steady input voltage vs. currentcharacteristic shown in FIG. 3. The charger controller's input voltageas sourced by the adapter 10 may be slightly higher than the nominal(floating) voltage of the battery to be charged, and remains effectivelyconstant over an operating current range set by the charger controller.

The output of the adapter 10 is coupled to a controlled current flowpath circuit, such as, but not limited to, a bipolar transistor orMOSFET, shown at 21, the source-drain path through which current flowsfrom the adapter 10 to the battery 30 being charged. A control circuit25 for controlling the operation of the current flow path circuit 21 hasa current sense link 26 (which may be a sense, resistor) which monitorsthe current through the current flow path circuit 21, as well as avoltage sense link 27 coupled to monitor the voltage of battery 30 as itis charged. The control circuit 25 typically comprises conventionallyemployed threshold sensor and comparator-based control components of thetype used in a variety of current, voltage, and switching controlapplications.

The operation of the linear charger of FIG. 2 may be readily explainedwith reference to the waveforms shown in FIGS. 4, 5 and 6. At thebeginning of the charging cycle, the battery voltage shown at V_(BAT) inFIG. 4 is at some less-than-nominal value, V_(BAT0). With the MOSFET 21being rendered conductive by control circuit 25, a prescribed constantcharging current I_(CHG) flows through the MOSFET's source-drain pathfrom the adapter 10 and into battery 30. As shown in FIG. 5, thisregulated charging current continues to flow up to the point at whichthe battery voltage reaches its floating (nominal) voltage V_(BATNOM).Once the battery voltage reaches its nominal voltage, the controlcircuit 25 regulates the battery voltage at this target value, causingthe current flowing in the MOSFET 21 to slowly decrease until completionof the charge. As will be appreciated from the foregoing description,and as shown in FIGS. 4 and 5 in particular, a typical linear batterycharger exhibits a constant current (FIG. 5)-constant voltage (FIG. 4)charge profile.

In order to match an increase in cell capacity, the charging currentneeds to increase. However, as shown in FIG. 6, it suffers fromsubstantial thermal dissipation, due to higher charging current. Inparticular, at the beginning of a recharging cycle a ‘fully’ dischargedbattery may exhibit a voltage on the order of 2.5 VDC, and a typicalfloating voltage value is on the order of 4.2 VDC. If, for example, theinput voltage is selected to be 5.0 VDC (which is only 800 MV above the4.2 V floating voltage) and the battery charging current is one ampere,the thermal dissipation will be (5 V−2.5 V)×1A=2.5 W at the beginning ofthe charging cycle.

One approach to reduce the thermal dissipation is to employ a pulsecharger, such as that illustrated in FIG. 7, which is similar to thelinear charger of FIG. 2, except that there is no current sense link,the current limiting function being built into the adapter, as shown bythe voltage vs. current relationship of FIG. 8. The operation of a pulsecharger may be understood by reference to the diagrams of FIGS. 9, 10and 11. During constant current mode (FIG. 9), the control circuit 25fully turns on the current flow/pass element (MOSFET) 21. As a result,the voltage across the pass element will be either a saturation voltage(if element 21 is a bipolar transistor) or, in the FIG. 7 example ofusing a MOSFET, will be the product of the charging current and ONresistance R_(ON) of MOSFET 21.

As shown in FIG. 8, the adapter 10 operates in a constant current regionand its output voltages collapses to a voltage slightly higher than thebattery voltage. Thus, the charger does not need to control the chargingcurrent, which is limited by the adapter (the charging current source).The thermal dissipation associated with a pulse type of charger is theproduct of the voltage across the pass element 21 and the chargingcurrent. For example, if the charging current is one ampere, as in thelinear case, described above, and the ON resistance R_(ON) of the passelement (MOSFET) is 300 milliohms, then the power dissipation will be0.3 Ohm×1A×1A=300 mW, a much smaller value than 2.5 W for the case of alinear charger described above.

As shown in FIG. 10, as the battery voltage approaches the floating ornominally fully charged battery voltage, the pulse charger starts toalternately turn the pass element (MOSFET 21) on and off, and graduallyreduces the duty ratio of the ON time, until termination of the chargingcycle. Power dissipation (shown in FIG. 11) is 300 mW when the passelement is on and zero when it is off. Therefore, the averagedissipation is less than 300 mW during the pulse phase.

Although low power dissipation is a principal advantage of a pulsecharger, such a charger requires a particular type of adapter—i.e., acurrent-limiting adapter. The main disadvantage of a pulse charger isthe fact that, during pulse mode operation, it produces pulsed voltagesat both the input and output of the charger, which constitute potentialelectromagnetic interference (EMI) noise that may affect the operationof one or more electronic circuits in the device powered by the batterybeing charged. In addition, the pulse charger may affect the lifetime ofthe battery and is not recommended by most battery cell manufacturers.

A third type of charger that may be employed is a switching charger. Aswitching type charger requires more components (including a bulkyoutput inductor) and switches large currents at high speeds, making itthe most noisy and complicated among the three types of chargers. It ismost practical for high-current applications, such as notebookcomputers.

SUMMARY OF THE INVENTION

In accordance with the present invention, the above discussed drawbacksof conventional battery charger topologies operating from a plug-inadapter are effectively obviated by a multi-adapter-compatible batterycharger controller as described herein. The present invention has thesame general circuit topology as the linear charger of FIG. 1 describedabove, but differs in respect to the operation of its control circuitwhen powered from a current-source adapter. Upon initialization of acharge sequence with a current-source adapter and a discharged battery,the adapter voltage will first rise to its compliance voltage leveluntil the charge controller demands a fast charge current to thebattery.

Upon initialization of the fast charge mode, the charge controller willdemand the full current limit set by the controller but will not be ableto provide that amount of current because the external adapter iscurrent limited to a value less than the controller-set level. Thecontroller will therefore turn on its control pass element (such as aMOSFET) to minimize the resistance between the adapter output and thebattery that is being charged, thereby forcing the adapter to enter itscurrent limit state and consequently bringing its voltage very close tothe battery voltage. It maintains that reduced voltage level whilesustaining the current limit condition.

The controller UVLO (under-voltage-lock-out) level is lower than for aconventional charger to accommodate the reduced current-limited adapteroutput voltage. The reduced voltage differential across the controllerpass element when the adapter is in current-limit translates intoreduced power dissipation as compared to a constant voltage adapterinput during this constant current charge phase. The controller willmaintain the charge current at this adapter current limit level—with theadapter voltage slightly higher than the battery voltage—until thebattery voltage attains its float voltage level. When the batteryreaches its float voltage level, the controller will start activelyregulating the battery voltage to maintain the prescribed float voltagewhile reducing the current required of the adapter.

As soon as the controller reduces its current demand from the adapter,the adapter output voltage will very quickly rise (“snap back”) to itscompliance level as it reverts to its voltage mode regulation.Coincident with this voltage “snap back” is an increase in instantaneouspower dissipation (FIG. 15) because the charger current is just slightlyreduced from its maximum level while the voltage differential hasincreased significantly. This dissipation will essentially track thedecay in current during the constant voltage phase unless thedissipation is high enough to trigger the controller into its thermalregulation mode.

If the temperature of the controller rises to a prescribed threshold itwill begin decreasing the charge current to lower the dissipation andtherefore the temperature. This assures that the controller does notabruptly interrupt the charge, as is characteristic of more conventionalcontroller types, but simply moderates the charge rate to a thermallymanageable level. This hybrid charging protocol of the present inventionyields recharge times comparable to a pulse mode controller andtypically faster recharge than a simple constant voltage adapterprotocol with the same current limits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically illustrates a conventional single-cell batterycharger;

FIG. 2 diagrammatically illustrates the general circuit architecture ofa linear battery charger;

FIG. 3 shows the substantially steady input voltage vs. currentcharacteristic of the linear battery charger of FIG. 2;

FIG. 4 shows the output voltage vs. charging time characteristic of thelinear battery charger of FIG. 2;

FIG. 5 shows the charging current vs. time characteristic of the linearbattery charger of FIG. 2;

FIG. 6 shows relative power dissipation vs time characteristic of thelinear battery charger of FIG. 2;

FIG. 7 diagrammatically illustrates the general architecture of a pulsedbattery charger;

FIG. 8 shows the input voltage vs. current characteristic of the pulsedbattery charger of FIG. 7;

FIG. 9 shows the charging current vs. time characteristic of the pulsedbattery charger of FIG. 7;

FIG. 10 shows the output voltage and input voltage vs. charging timecharacteristic of the pulsed battery charger of FIG. 7;

FIG. 11 shows relative power dissipation vs time characteristic of thepulsed battery charger of FIG. 7;

FIG. 12 diagrammatically illustrates the general architecture of abattery charger in accordance with the invention;

FIG. 13 shows the charging current vs. time characteristic of thebattery charger of FIG. 12;

FIG. 14 shows the output voltage and input voltage vs. charging timecharacteristic of the battery charger of FIG. 12;

FIG. 15 shows relative power dissipation vs time characteristic of thebattery charger of FIG. 12;

DETAILED DESCRIPTION

Before describing the multi-adapter compatible battery charger inaccordance with the invention, it should be observed that the inventionresides primarily in an arrangement of conventional DC power supplycircuits and control components integrated together. It is to beunderstood that the invention may be embodied in a variety ofimplementations, and should not be construed as being limited to onlythose shown and described herein. For example, although the non-limitingcircuit diagrams of the Figures shows the use of MOSFET devices toperform controlled current path operations, it will be appreciated thatthe invention is not limited thereto, but also may be configured ofalternative equivalent circuit devices, such as, bipolar transistors.The implementation example to be described is intended to furnish onlythose specifics that are pertinent to the present invention, so as notto obscure the disclosure with details that are readily apparent to oneskilled in the art having the benefit of present description. Throughoutthe text and drawings like numbers refer to like parts.

Attention is now directed to FIG. 12, which diagrammatically shows anexample of an embodiment of a multi-adapter compatible battery chargercontroller in accordance with the invention. As shown therein, thebattery charger controller of the invention has the same general circuittopology as the linear battery charger of FIG. 1. As shown therein thecontrol circuit has a first input terminal coupled to ground and a firstoutput terminal coupled to ground. A source of DC voltage 10 isreferenced to ground and has its +output terminal coupled to a secondinput terminal of the control circuit. A second output terminal of thecontrol circuit is coupled to the voltage sensing line 27 and the+terminal of battery 30. Also shown in broken lines is a DC-DC converteror power adapter with its DC output port coupled to the second inputterminal. Also shown in broken lines is an AC-DC converter which has theAC input port thereof coupled to an AC power outlet. The principaldifference between the multi-adapter compatible battery chargercontroller of FIG. 12 and the linear battery charger of FIG. 1 involvesthe operation of the control circuit 25 when the adapter's constantvoltage output transitions to a value that limits the adapter'savailable charging current to a value I_(LIM) (less than the programmedconstant charging current I_(REF),). As in the above described linearand pulsed charger circuits, the control circuit 25 employed in thecharger of FIG. 12 also uses conventional threshold sensor andcomparator-based control components for current and voltage control andswitching applications. Rather than detail those components, the presentdescription will describe the input and response parameters employed forexecuting the control functionality that enable the control circuit toprovide the augmented charger capabilities of the invention.

Up to the transition point between constant current mode and constantvoltage mode, the battery voltage has been gradually increasing, asshown in FIG. 14, from some nominal value V_(BATNOM), and approaching afloating fully charged battery voltage. Upon reaching thecurrent-limiting threshold, the adaptor maintains the charge current atI_(LIM), as shown in FIG. 8, and also by the dotted lines 131-132 ofFIG. 13. Since this current (which is monitored by the control circuit25) is less than the programmed reference current I_(REF), the controlcircuit 25 responds by enhancing the current throughput of thecontrolled current flow path circuit which, in the illustratedembodiment, corresponds to an increase in the gate drive to MOSFET 21,so that MOSFET 21 is fully turned on. With MOSFET 21 fully turned on,power dissipation in the charger controller is considerably reduced incomparison to the linear charger operation, as can be seen from acomparison of FIG. 15 with FIG. 6.

As pointed out above, a principal reason that the multipleadapter-compatible charger controller of the present invention is ableto operate at high current without large thermal dissipation is thereduced under-voltage lockout (UVLO) level employed. As the batteryvoltage reaches the floating voltage (FIG. 14), the charger controllercurrent decreases to the adapter's limit value (shown in dotted lines131 and 132 in FIG. 13). When the adapter starts to operate in theconstant voltage region as shown in FIG. 14, the charger controllerinput voltage jumps from a voltage 141 that was slightly higher than thebattery floating voltage to a new constant output voltage 142 of theadapter, that is higher than the floating voltage value.

Once it has transitioned to this constant voltage mode, the chargercontroller operates in substantially the same manner as a linearcharger. At the very beginning of this constant voltage mode, the valueof the charger controller current is still fairly large, which couldlead to a fairly large power dissipation. However, due to the use ofthermal foldback of the charge current, the charger currentcharacteristic of FIG. 13 undergoes a sharply (stepwise) reduced currentvalue to a level 132, such that the thermal dissipation does not exceedthe limit set by the charger controller, as shown with the dotted lines151 in the power dissipation of FIG. 15. During this reduced chargingcurrent interval shown by the boundary lines 135, the charger currentvalue 132 is substantially less than that (curve 133) of a linearcharger, described above, and shown in FIG. 5.

While this reduced current flow interval causes a small increase inbattery charge time, it occupies only a small fraction of the overallcharge time, and therefore has no practical disadvantage. As thecharging current continues to decrease, as shown by curve 134 in FIG.13, the power dissipation reduces further to a value less than thethermal limit, as shown at curve 154 in FIG. 15. Therefore, the chargercontroller of the invention completes in charging operation insubstantially the same manner as a conventional linear charger.

From the above description, it will be appreciated that the chargercontroller of the present invention has the same thermal performance asa pulse charger, except during the period that the charger operates atits thermal limit. The thermal limit automatically regulates the chargecurrent to a level that the entire operation of the charger circuit isthermally safe. If a voltage source adapter is plugged in, the chargercontroller of the invention operates the same as a linear charger. Theonly difference is that if the power dissipation in the charger exceedsthe thermal limit, the charger controller will automatically reduce thecharging current, so that the circuit is thermally safe.

In addition, once designed for incorporation into a given application,the battery charging circuit of the invention is safe to be used withany popular type of adapter as the power source with correct voltagepolarity and range. This type of commonly used low-cost unregulatedadapter usually consists of a step-down transformer, a rectifier, and anoutput filtering capacitor.

While we have shown and described an embodiment in accordance with thepresent invention, it is to be understood that the same is not limitedthereto, but is susceptible to numerous changes and modifications asknown to a person skilled in the art. We therefore do not wish to belimited to the details shown and described herein, but intend to coverall such changes and modifications as are obvious to one of ordinaryskill in the art.

1. An apparatus for charging a battery comprising: a first inputterminal; second input terminal; a first output terminal arranged to becoupled to a reference voltage, and to a first terminal of said battery;a second output terminal arranged to be coupled to a second terminal ofsaid battery; a controlled current flow path between said second inputterminal and said second output terminal; and a control circuit which isoperative, in response to a DC power source being coupled to said firstand second input terminals, to cause said controlled current flow pathto provide a first, substantially constant battery charging currenttherethrough, so as to charge said battery to a prescribed voltage and,in response to a voltage of said battery reaching said prescribedvoltage, to effect a stepwise reduction in the battery charging currentto a second, substantially constant battery charging current less thansaid first, substantially constant battery charging current for a periodof time, such that thermal dissipation does not exceed a limit set bysaid control circuit, and thereafter gradually reducing battery chargingcurrent, so as to reduce power dissipation to a value less than athermal limit.
 2. The apparatus according to claim 1, wherein saidcontrol circuit is further operative, in response to the voltage of saidbattery reaching said prescribed voltage, to effect a stepwise increasein input voltage to a constant voltage higher than said prescribedvoltage for the duration of said stepwise reduction in the batterycharging current to said second, substantially constant battery chargingcurrent less than said first, substantially constant battery chargingcurrent, and for a time thereafter during which said battery chargingcurrent is gradually reduced so as to reduce power dissipation to saidvalue less than said thermal limit.
 3. The apparatus according to claim1, wherein said DC power source comprises an AC-DC converter that isarranged to be connected to an AC power outlet, and having a DC outputports that are arranged to be coupled to said first and second inputterminals.
 4. The apparatus according to claim 1, wherein said controlcircuit is operative, in association with said stepwise reduction in thebattery charging current to a second, substantially constant batterycharging current less than said first, substantially constant batterycharging current for said period of time, to increase turn on drive tosaid controlled current flow path, so that power dissipation in thecontrol circuit is considerably reduced in comparison to a linearcharging operation.
 5. The apparatus according to claim 1, wherein saidcontrolled current flow path comprises a controlled electronic circuitdevice having an input-output current flow path coupled between saidsecond input terminal and said second output terminal, and a controlterminal arranged to receive a control input from said control circuitthat controls current throughput of said input-output current flow pathof said controlled electronic circuit device.
 6. The apparatus accordingto claim 5, wherein said controlled electronic circuit device comprisesa MOSFET, and said control circuit is operative, in association withsaid stepwise reduction in the battery charging current to a second,substantially constant battery charging current less than said first,substantially constant battery charging current for said period of time,to increase gate drive to said MOSFET so as to place said MOSFET in afully turned on condition.
 7. The apparatus according to claim 1,wherein said battery comprises an Li-ion/Li-polymer battery.
 8. Theapparatus according to claim 1, wherein said control circuit exhibits areduced under-voltage-lock-out level that is lower than the minimalexpected battery voltage, to accommodate reduced current-limited adapteroutput voltage.
 9. An apparatus for charging a battery to a prescribedbattery voltage comprising first and second input terminals arranged tobe coupled to DC output terminals of an AC adapter containing an AC-DCconverter, and first and second output terminals arranged to be coupledwith first and second terminals of said battery, a controlled currentflow path coupled between said first input terminal and said firstoutput terminal, and a control circuit which is operative to cause saidcontrolled current flow path to provide a first, substantially constantbattery charging current therethrough, so as to charge said battery to aprescribed voltage and, in response to a voltage of said batteryreaching said prescribed voltage for a constant voltage mode ofoperation, to effect a thermal foldback-based stepwise reduction in thebattery charging current to a second, substantially constant batterycharging current less than said first, substantially constant batterycharging current for a period of time, such that thermal dissipationdoes not exceed a limit set by said control circuit, and thereaftergradually reducing battery charging current, so as to reduce powerdissipation to a value less than a thermal limit.
 10. The apparatusaccording to claim 9, wherein said control circuit is further operative,in response to the voltage of said battery reaching said prescribedvoltage, to effect a stepwise increase in input voltage to a constantvoltage higher than said prescribed voltage for the duration of saidstepwise reduction in the battery charging current to said second,substantially constant battery charging current less than said first,substantially constant battery charging current, and for a timethereafter during which said battery charging current is graduallyreduced so as to reduce power dissipation to said value less than saidthermal limit.
 11. The apparatus according to claim 9, wherein saidcontrol circuit is operative, in association with said stepwisereduction in the battery charging current to a second, substantiallyconstant battery charging current less than said first, substantiallyconstant battery charging current for said period of time, to increaseturn on drive to said controlled current flow path, so that powerdissipation in the control circuit is considerably reduced in comparisonto a linear charging operation.
 12. The apparatus according to claim 9,wherein said controlled current flow path comprises a controlledelectronic circuit device having an input-output current flow pathcoupled between said second input terminal and said second outputterminal, and a control terminal arranged to receive a control inputfrom said control circuit that controls current throughput of saidinput-output current flow path of said controlled electronic circuitdevice.
 13. The apparatus according to claim 12, wherein said controlledelectronic circuit device comprises a MOSFET, and said control circuitis operative, in association with said stepwise reduction in the batterycharging current to a second, substantially constant battery chargingcurrent less than said first, substantially constant battery chargingcurrent for said period of time, to increase gate drive to said MOSFETso as to place said MOSFET in a fully turned on condition.
 14. Theapparatus according to claim 9, wherein said control circuit exhibits areduced under-voltage-lock-out level that is lower than the minimalexpected battery voltage, to accommodate reduced current-limited adapteroutput voltage.
 15. The apparatus according to claim 9, wherein saidbattery comprises an Li-ion/Li-polymer battery.
 16. A method forcharging a battery to a prescribed battery voltage comprising the stepsof: (a) coupling an AC adapter containing an AC-DC converter to a sourceof AC power, said AC adapter having first and second output terminalsfrom which a DC voltage is supplied; (b) coupling first and second inputterminals of a battery charger circuit to said first and second outputterminals of said AC adapter, said battery charger circuit including acontrolled current flow path coupled between said first input terminaland a first output terminal of said battery charger circuit; (c)coupling a battery to be charged to first and second output terminals ofsaid battery charger circuit; and (d) causing said controlled currentflow path of said battery charger circuit to provide a first,substantially constant current therethrough from said AC adapter to saidbattery and charge said battery to said prescribed battery voltage and,in response to a voltage of said battery reaching said prescribedvoltage for a constant voltage mode of operation, to effect a thermalfoldback-based stepwise reduction in the battery charging current to asecond, substantially constant battery charging current less than saidfirst, substantially constant battery charging current for a period oftime, such that thermal dissipation does not exceed a limit set by saidcontrol circuit, and thereafter gradually reducing battery chargingcurrent, so as to reduce power dissipation to a value less than athermal limit.
 17. The method according to claim 16, wherein step (d)further comprises, in response to the voltage of said battery reachingsaid prescribed voltage, effecting a stepwise increase in input voltageto a constant voltage higher than said prescribed voltage for theduration of said stepwise reduction in the battery charging current tosaid second, substantially constant battery charging current less thansaid first, substantially constant battery charging current, and for atime thereafter during which said battery charging current is graduallyreduced so as to reduce power dissipation to said value less than saidthermal limit.
 18. The method according to claim 16, wherein said step(d) comprises, in association with said stepwise reduction in thebattery charging current to said second, substantially constant batterycharging current less than said first, substantially constant batterycharging current for said period of time, increasing turn on drive tosaid controlled current flow path, so that power dissipation isconsiderably reduced in comparison to a linear charging operation. 19.The method according to claim 16, wherein said controlled current flowpath comprises a controlled electronic circuit device having aninput-output current flow path coupled between said second inputterminal and said second output terminal, and a control terminalarranged to receive a control input that controls current throughput ofsaid input-output current flow path of said controlled electroniccircuit device.
 20. The method according to claim 19, wherein saidcontrolled electronic circuit device comprises a MOSFET, and step (d)comprises, in association with said stepwise reduction in the batterycharging current to a second, substantially constant battery chargingcurrent less than said first, substantially constant battery chargingcurrent for said period of time, increasing gate drive to said MOSFET soas to place said MOSFET in a fully turned on condition.
 21. An apparatusfor charging a battery comprising: a first input terminal; a secondinput terminal; a first output terminal arranged to be coupled to areference voltage, and to a first terminal of said battery; a secondoutput terminal arranged to be coupled to a second terminal of saidbattery; a controlled current flow path between said second inputterminal and said second output terminal; and a control circuit which isoperative to charge said battery by way of said first and second outputterminals, when any of a plurality of different types of DC powersources are coupled to said first and second input terminals.
 22. Theapparatus according to claim 21, wherein one of said plurality ofdifferent types of DC power sources comprises a constant current powersource, and wherein another of said plurality of different types of DCpower sources comprises a constant voltage—constant currant powersource.
 23. An apparatus for charging a battery comprising: a firstinput terminal; a second input terminal; a first output terminalarranged to be coupled to a reference voltage, and to a first terminalof said battery; a second output terminal arranged to be coupled to asecond terminal of said battery; a controlled current flow path betweensaid second input terminal and said second output terminal; and acontrol circuit which is operative to charge said battery by way of saidcontrolled current flow path and said first and second output terminals,said control circuit having a reduced undervoltage lockout level that islower than the minimal expected battery voltage and employing thermalfoldback of the charging current.